1. Field of the Invention
The present invention relates to a super heat-resistant silicon carbide fiber having a high heat resistance particularly in an oxygen-free atmosphere, and a process for producing the same. More particularly, the present invention is concerned with a silicon carbide fiber which is produced from an organosilicon polymer compound such as polycarbosilane and has a high strength, a high modulus of elasticity and a very high heat resistance particularly in an oxygen-free atmosphere, and a process for producing the same.
2. Prior Art
The process for producing a silicon carbide fiber through the use of an organosilicon polymer compound as a precursor generally comprises three steps of spinning, infusibilization and firing.
Among these steps, the spinning step is one for preparing a precursor fiber having a small diameter through the melt spinning of an organosilicon polymer compound such as polycarbosilane.
In the infusibilization step, the precursor fiber undergoes a crosslinking reaction so that the fiber is made insoluble and infusible in the subsequent step of firing. In the prior art, the precursor fiber has been generally heat-treated in an oxidizing atmosphere such as air to bring about the crosslinking reaction (hereinafter referred to as "thermal oxidation process").
In the firing step, the infusibilized fiber is converted into an inorganic compound to give a silicon carbide fiber. In the prior art, the infusibilized fiber has been generally treated at a high temperature of about 1200.degree. C. in an oxygen-free atmosphere or in vacuo.
The silicon carbide fiber will find applications mainly as a composite material (FRM) comprising the silicon carbide fiber and a metal, and a composite material (FRC) comprising the silicon carbide fiber and a ceramic in the future. In order to use the silicon carbide fiber for such applications, the silicon carbide fiber should have a high heat resistance at least in an oxygen-free atmosphere.
As described above, in the prior art, however, since the precursor fiber is made infusible through the crosslinking reaction of an organosilicon polymer compound by means of oxygen, the resultant silicon carbide fiber is a ternary composition comprising silicon, carbon and oxygen and containing 8 to 20% by weight of oxygen. In such a silicon carbide fiber, a defect occurs in the fiber in an oxygen-free atmosphere at a high temperature of 1300.degree. C. or above through the elimination of CO and SiO gases according to the following formulae (1) and (2): EQU SiO.sub.2 +3C.fwdarw.SiC+2CO.uparw. (1) EQU Si+C+O.fwdarw.1/2CO.uparw.+1/2SiO.uparw.+1/2SiC (2)
Further, the formation of SiC causes silicon carbide crystal grains to be coarsened. For this reason, the silicon carbide fiber prepared by the above-described conventional process exhibits a poor heat resistance due to a remarkable deterioration of the fiber strength particularly at 1500.degree. C. or above.
Other known methods of making the precursor fiber infusible include one wherein the precursor fiber is irradiated with a radiation in an atmosphere containing oxygen, such as air (see J. P. Appln. Laid-Open Gazette No. (Sho.) 53-103025). Even when this method is employed, the resultant silicon carbide fiber contains 7 to 30% by weight of oxygen as in the above-described thermal oxidation process and therefore is poor in heat resistance.
Further, some of the present inventors have disclosed a method wherein the precursor fiber is irradiated with a radiation in an oxygen-free atmosphere or in vacuo (see Proc. 1st Japan Int. SAMPE Symp., pp. 929-934, Nov. 28-Dec. 1, 1989). In the prior art, however, the fiber irradiated with a radiation for making the fiber infusible is placed as such in the air, afterwhich the infusibilized fiber is fired. Therefore, there is a limitation on the reduction of the oxygen content of the resultant silicon carbide fiber, and the silicon carbide fiber is unsatisfactory in heat resistance. Further, the above-described literature discloses that the oxygen content in the silicon carbide fiber is reduced when the infusibilized fiber is annealed by heating at about 250.degree. C. before being fired. In this method, however, the oxygen content in the silicon carbide fiber can be reduced to only 1 to 2% by weight.